Method for removing chlorate from alkali metal hydroxide

Abstract

A method is disclosed for reducing alkali metal chlorate impurities in an aqueous alkali metal hydroxide solution by heating a mixture of the alkali metal hydroxide solution with a hydroxycarboxylic acid or a salt thereof at temperature above about 125° C. and for a period of time sufficient to substantially eliminate the chlorate, the hydroxycarboxylic acid or salt thereof being substantially non-volatile at the heating temperature.

I claim: 
     
       1. A process for reducing chlorate concentration in a chlorate-containing aqueous alkali metal hydroxide solution, said process consisting of heating the alkali metal hydroxide solution in the presence of an effective amount of hydroxycarboxylic acid or sodium, potassium or ammonium salt thereof at a temperature of from at least 125° C. to about 190° C. for a time sufficient to substantially reduce the chlorate concentration in the alkali metal hydroxide solution, with the provisos that the hydroxycarboxylic acid or salt thereof is soluble in the alkali metal hydroxide solution and that the heating temperature is below the boiling point and decomposition temperature of the hydroxycarboxylic acid. 
     
     
       2. The process of claim 1 wherein the hydroxycarboxylic acid is selected from the group consisting of monohydroxymonocarboxylic acid, monohydroxydicarboxylic acid, monohydroxytricarboxylic acid, dihydroxymonocarboxylic acid and dihydroxydicarboxylic acid. 
     
     
       3. The process of claim 2 wherein the monohydroxymonocarboxylic acid is selected from the group consisting of hydroxyethanoic acid, 2-hydroxybutanoic acid, 4-hydroxybutanoic acid, o-hydroxybenzoic acid, m-hydroxybenzoic acid, p-hydroxybenzoic acid, hydroxy-(phenyl)ethanoic acid, and hydroxymethylbenzoic acid. 
     
     
       4. The process of claim 3 wherein the monohydroxymonocarboxylic acid is hydroxyethanoic acid. 
     
     
       5. The process of claim 2 wherein the monohydroxytricarboxylic acid salt is sodium citrate. 
     
     
       6. The process of claim 2 wherein the dihydroxydicarboxylic acid is 2,3-dihydroxybutanedioic acid. 
     
     
       7. The process of claim 2 wherein the dihydroxydicarboxylic acid salt is sodium tartrate. 
     
     
       8. The process of claim 1 wherein the effective amount of hydroxycarboxylic acid is at least six times the stoichiometric amount theoretically required to react with the chlorate. 
     
     
       9. The process of claim 1 wherein the temperature is at least about 140° C. 
     
     
       10. A process for reducing sodium chlorate concentration in a sodium chlorate-containing aqueous sodium hydroxide solution containing about 50 percent by weight sodium hydroxide, said process consisting of heating the sodium hydroxide solution in the presence of hydrocarboxylic acid or sodium, potassium or ammonium salt thereof at a temperature of from at least 125° C. to about 190° C. for a time sufficient to substantially reduce the sodium chlorate concentration in the sodium hydroxide solution, with the provisos that the hydroxycarboxylic acid or salt thereof is soluble in the sodium hydroxide solution and that the heating temperature is below the melting point and decomposition temperature of the hydroxycarboxylic acid. 
     
     
       11. The process of claim 10 wherein the hydroxycarboxylic acid or salt thereof is selected from the group consisting of hydroxyethanoic acid, sodium citrate and sodium tartrate. 
     
     
       12. The process of claim 10 wherein the heating temperature is at least about 140° C. 
     
     
       13. The process of claim 10 wherein the effective amount of hydroxycarboxylic acid is at least six times the stoichiometric amount theoretically required to react with the sodium chlorate.